QTR-Quadtone RIP

So I've set up this 7000 running QTR and the Piezography K6 inks. It works perfectly except that I notice a somewhat steep falloff in the midrange grayscale. Rather than explain verbally, see this image:

http://farm3.static.flickr.com/2733/4285702180_84bb9c300e_o.jpg

It compares two prints of the same 0-100% grayscale file, printed on the 7000 and on a 3800. All settings identical. Both printed with QTR and stock profiles on Enhanced Matte.

Note the difference in the mid-ranges. You'll see that while the 3800 prints the transition smoothly, the 7000 has a sharper falloff. 

Any ideas what would explain that performance, and what's the proper way to adjust the 7000? Many thanks for any diagnosis!